The present invention relates to a semiconductor laser module and a coupling lens used therein. Particularly, the present invention relates to a low power semiconductor laser module and a coupling lens that are suitable for a short-distance optical communication system.
In a semiconductor laser module used for optical communication, it is required to couple a semiconductor laser or receiving optics and an optical fiber efficiently. FIG. 23 shows a configuration of a conventional laser module. A semiconductor laser 2302 and a coupling lens 2303 are fixed to a lens holder 2301. The lens holder 2301 is inserted into a connecting holder 2304. To this connecting holder 2304, a ferrule holder 2307 is fixed. An optical fiber 2305 is fixed removably by a ferrule 2306. The coupling lens 2303 is made of glass. Abeam of light emitted from the semiconductor laser 2302 is focused on an end face of the optical fiber 2305 by the coupling lens 2303, thus being coupled to the fiber.
In this case, from a safety aspect in handling, it is necessary to restrict the optical output from the laser module to a certain level or lower. Therefore, in addition to the basic configuration shown in FIG. 23, the laser module is provided with: a means for reducing the optical output, such as an attenuation film, a polarizer, or the like, between the semiconductor laser and the optical fiber (for instance, JP 4-97208 A, JP 7-43563 A, or the like); an aperture to control the quantity of light; or a control circuit for stopping emission of the semiconductor laser automatically when the optical fiber comes off.
As a condenser lens, an aspheric lens made of glass has been used conventionally, but for the purpose of cost reduction, a resin lens has come to be used (for instance, JP 5-60952 A, JP 61-245594 A, JP 5-27140 A, JP 5-60940, or the like). In the case of using the resin lens, a refractive index of resin varies with variation in temperature, resulting in variation in the focal length. In addition, its coefficient of thermal expansion is higher than that of a glass material. Therefore, an imaging position varies with variation in temperature, resulting in variation in the coupling efficiency to an optical fiber.
In the semiconductor laser module, an emission wavelength of the semiconductor laser as a light source also varies with variation in temperature. Therefore, it is conceivable that by providing a diffraction lens on the surface of the lens, the variation of focal length of the resin lens with temperature is corrected by the diffraction lens. The reason is that since a focal length of the diffraction lens varies greatly depending on a wavelength of the light source compared to that of a refractive lens, the focal length of the diffraction lens varies more than that of the refractive lens when the wavelength of the light source varies with variation in temperature. In other words, by employing a design enabling the variation in focal length of the diffraction lens to cancels out the variation of focal length with the temperature of the refractive lens, a lens in which the variation in focal length due to variation in temperature has been compensated can be obtained.
In the above-mentioned method of using an attenuation film, a polarizer, or the like for restricting the optical output from the laser module to a certain level or lower, manufacturing cost is high. In the method of controlling the quantity of light by an aperture, the quantity of light varies depending on the processing accuracy of the aperture or variation in flare angle of the laser, and in addition, high assembly accuracy is required. Further, the method of providing a control circuit for stopping emission of the laser automatically when the optical fiber comes off is not preferable, since not only the configuration of a device as a whole becomes complicated but also the manufacturing cost is high.
When a resin lens is used for reducing a lens cost, a diffraction lens is formed on the surface of the lens so as to correct the variation in focal length due to the variation in temperature since the variation in refractive index due to variation in temperature is greater in a resin lens compared to that in a glass lens. As a method of manufacturing such a one-piece lens in which a diffraction lens is integrated, a method of processing a lens or a mold for manufacturing a lens by precision cutting using a diamond bit has been used widely. In this case, since the tip of the diamond bit has finite roundness (a nose radius), a processed relief comes to have a shape with edges rounded due to the nose radius of the bit. In a diffraction lens, generally a sawtooth relief shape is used in many cases. However, when a sawtooth relief is processed by the above-mentioned cutting, the diffraction efficiency at the periphery of the lens deviates from a design value.
This is because a pitch of zones of the diffraction lens becomes shorter at the periphery of the lens, and therefore the influence of deterioration in the relief shape caused by the nose radius of the bit becomes more serious at the periphery compared to the center portion of the lens having a longer pitch. In a lens for optical fiber coupling, when the diffraction efficiency at the periphery of the lens decreases, the same effect as that in the case where an effective NA of the lens is decreased is provided and therefore a spot is enlarged, thus causing the decrease in coupling efficiency to an optical fiber. In order to prevent this, there is a method of using a bit for processing with a sharp tip, but it causes the decrease in productivity, which is therefore not preferable. In order to carry out the processing without impairing the productivity, it is said to be desirable that a bit has a nose radius of at least about 10 xcexcm. In addition, a diffraction lens for temperature compensation requires higher power (refracting power) than that of a diffraction lens for so-called chromatic-aberration correction and as a result, has a shorter pitch of diffraction zones at the periphery of the lens compared to the diffraction lens for chromatic-aberration correction. Therefore, in a conventional diffraction lens having a sawtooth relief shape, there has been a problem that the compatibility between the productivity of the lens and mold and the diffraction efficiency performance cannot be obtained.
When using a diffraction lens, a plurality of focal spots corresponding to respective diffraction orders are generated on an optical axis. In order to reduce the intensity of light emitted from a module, an aperture is provided at a position of a focal spot corresponding to a diffraction order used for coupling, thus intercepting lights focused on focal spots corresponding to unnecessary diffraction orders. In this case, lights focused on focal spots with longer focal lengths than that of a focal spot corresponding to the diffraction order used for coupling tend to go through the aperture compared to lights focused on focal spots with shorter focal lengths. In the case of a conventional diffraction lens with a sawtooth relief, when its shape is varied by cutting, unwanted lights tend to be focused on the focal spots corresponding to lower orders and cannot be intercepted by the aperture completely, which has been a problem.
In view of the above points, the present invention is intended to provide an inexpensive optical coupler in which the diffraction efficiency of a diffraction lens formed on a surface of a lens is adjusted and therefore an attenuation film or a polarizer for restricting optical output from a laser module to a certain level or lower, or a control circuit for stopping emission automatically when an optical fiber comes off is not required.
In order to achieve the above-mentioned object, the present invention employs the following configurations.
A coupling lens according to a first configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optical fiber. The coupling lens is formed of a single lens. On either one of an incident plane and an outgoing-side plane of the single lens, a diffraction lens formed of concentric zones is integrated. The diffraction lens has a positive refracting power, and a relief function of the diffraction lens has an approximately isosceles triangular shape. The depth W of the relief function satisfies the following formula:
0.6xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa61.0.
Preferably, it satisfies the following formula:
0.6 less than W(nxe2x88x921)/xcex less than 1.0.
In the above formulae, W indicates the depth of the relief function, n denotes a refractive index of a lens material, and xcex represents a wavelength of the semiconductor laser.
The coupling lens according to the first configuration has a relief with an approximately isosceles triangular shape and therefore has excellent productivity even when manufactured by a mold processed by cutting using a diamond bit. The coupling lens has approximately uniform diffraction efficiency from its center to periphery and thus has excellent focusing performance.
A coupling lens according to a second configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optical fiber. The coupling lens is formed of a single lens. On either one of an incident plane and an outgoing-side plane of the single lens, a diffraction lens formed of concentric zones is integrated. The diffraction lens has a positive refracting power, and a relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 25% and 45% of a relief period. The depth W of the relief function satisfies the following formula:
0.9xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa61.2.
Preferably, it satisfies the following formula:
0.9 less than W(nxe2x88x921)/xcex less than 1.2.
In the above formulae, W indicates the depth of the relief function, n denotes a refractive index of a lens material, and xcex represents a wavelength of the semiconductor laser.
The coupling lens according to the second configuration has an approximately triangular wavy relief shape. Therefore, the coupling lens focuses unwanted light on focal spots corresponding to higher diffraction orders than that used for optical fiber coupling, thus decreasing leakage of unwanted light from an aperture for intercepting the unwanted light.
In the coupling lens according to the second configuration, it is preferable that the relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 30% and 40% of a relief period and the depth W of the relief function satisfies the following formula:
1.0xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa61.1
In the coupling lens according to the first or second configuration, it is preferable that first-order diffracted light from the diffraction lens is used for optical fiber coupling, and when the diffraction efficiency of zero-order diffracted light is xcex70 and the diffraction efficiency of second-order diffracted light is xcex72, the following formula is satisfied:
xcex70 less than xcex72.
Further, in the coupling lens according to the first or second configuration, it is preferable that the diffraction efficiency xcex70 of zero-order diffracted light from the diffraction lens satisfies the following formula:
xcex70 less than 7%.
A coupling lens according to a third configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optical fiber. The coupling lens is formed of a single lens. On either one of an incident plane and an outgoing-side plane of the single lens, a diffraction lens formed of concentric zones is integrated. The diffraction lens has a positive refracting power and is processed so as to be uncentered with respect to an axis of rotational symmetry of a refractive lens.
In the coupling lens of the third configuration, since the diffraction lens is processed so as to be uncentered with respect to the axis of rotational symmetry of the refractive lens, lights with different diffraction orders from a location of the refractive lens corresponding to its axis of rotational symmetry are focused on focal spots different both in focal length and image height. Therefore, unwanted light can be intercepted easily, thus relaxing processing accuracy and assembly accuracy of an aperture.
In each coupling lens of the first to third configurations, it is preferable that when the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction lens is xcex7, the following formula is satisfied:
25%xe2x89xa6xcex7xe2x89xa640%
and more preferably,
25% less than xcex7 less than 40%.
In each coupling lens of the first to third configurations, it is preferable that when the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction lens is xcex7, the following formula is satisfied:
30%xe2x89xa6xcex7xe2x89xa637%
and further preferably,
30% less than xcex7 less than 37%.
In each coupling lens of the first to third configurations, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
In each coupling lens of the first to third configurations, it is preferable that a material of the lens is resin, and the diffraction lens is designed so that when a refractive index of the resin material and a wavelength of the semiconductor laser vary due to variation in temperature, the variation in focal length of the lens due to variation in the refractive index is corrected by the variation in focal length of the diffraction lens due to variation in the wavelength.
Since the coupling lens has a configuration in which the diffraction lens is integrated so as to correct the variation in focal length caused by variation in temperature, inexpensive resin can be used as a material of the lens, thus reducing the cost of the lens.
In each coupling lens of the first to third configurations, it is preferable that a material of the lens is resin and when a focal length of the lens as a whole is f and a focal length of the diffraction lens is fd, the following formula is satisfied:
2 less than fd/f less than 5.
According to this, the above-mentioned temperature compensation function can be provided.
A semiconductor laser module according to a first configuration of the present invention includes, at least: a semiconductor laser; an optical fiber; a fixing member for fixing an incident end of the optical fiber; and a coupling lens for allowing a beam of light emitted from the semiconductor laser to form an image on the incident end of the optical fiber. The coupling lens is any one of the first to third coupling lenses.
In the semiconductor laser module of the first configuration, since any one of the above-mentioned coupling lenses of the present invention is used, when intercepted using an aperture, the unwanted light can be intercepted easily, thus relaxing processing accuracy and assembly accuracy of the aperture. Consequently, a semiconductor laser module can be provided at a low cost.
In the semiconductor laser module of the first configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
A semiconductor laser module according to a second configuration of the present invention includes, at least: a semiconductor laser; an optical fiber; a fixing member for fixing an incident end of the optical fiber; and a coupling lens for allowing a beam of light emitted from the semiconductor laser to form an image on the incident end of the optical fiber. The coupling lens is the coupling lens according to claim 1, 2 or 6 and is fixed so as to be tilted with respect to an optical axis.
In the semiconductor laser module of the second configuration, since the coupling lens of the present invention is fixed so as to be tilted with respect to the optical axis, focal spots of diffracted lights with orders that are not used for optical fiber coupling are generated at positions different both in focal length and image height. Therefore, the processing accuracy and assembly accuracy of an aperture for intercepting unwanted light can be relaxed. As a result, the semiconductor laser module can be manufactured at a low cost.
In the semiconductor laser module of the second configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
A coupling lens according to a fourth configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optical fiber. The coupling lens is made of glass and is formed of a single lens. On either one of an incident plane and an outgoing-side plane of the single lens, a diffraction grating is formed.
Since the coupling lens of the fourth configuration is made of glass and the diffraction grating is formed on a surface of the lens, unwanted diffracted light that is not used for coupling can be separated on an image surface.
In the coupling lens of the fourth configuration, it is preferable that zero-order diffracted light from the diffraction grating is used for optical fiber coupling and the diffraction efficiency xcex70 of the zero-order diffracted light satisfies the following formula:
25%xe2x89xa6xcex70xe2x89xa640%.
More preferably, the diffraction efficiency xcex70 satisfies the following formula:
25% less than xcex70 less than 40%.
Furthermore, in the coupling lens of the fourth configuration, it is preferable that zero-order diffracted light from the diffraction grating is used for optical fiber coupling and the diffraction efficiency xcex70 of the zero-order diffracted light satisfies the following formula:
30%xe2x89xa6xcex70xe2x89xa637%.
More preferably, the diffraction efficiency xcex70 satisfies the following formula:
30% less than xcex70 less than 37%.
A semiconductor laser module according to a third configuration of the present invention includes, at least: a semiconductor laser; an optical fiber; a fixing member for fixing an incident end of the optical fiber; and a coupling lens for allowing a beam of light emitted from the semiconductor laser to form an image on the incident end of the optical fiber. The coupling lens is the coupling lens of the fourth configuration.
In the semiconductor laser module of the third configuration, since the aforementioned coupling lens is used, the output power can be reduced to be within a safety standard without using a safety circuit or a specific coating. Thus, the semiconductor laser module can be manufactured at a low cost.
In the semiconductor laser module of the third configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
A coupling lens according to a fifth configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optical fiber. The coupling lens is formed of a single lens. On either one of an incident plane and an outgoing-side plane of the single lens, a diffraction lens formed of concentric zones is integrated. The diffraction lens has a positive refracting power. When the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction lens is xcex7, the following formula is satisfied:
25%xe2x89xa6xcex7xe2x89xa640%.
More preferably, the following formula is satisfied:
25% less than xcex7 less than 40%.
In the coupling lens of the fifth configuration, the diffraction efficiency is designed considering the power of the light source and losses in the quantity of light in the lens and in the fiber coupling. Therefore, when the coupling lens is used in a semiconductor laser module, the output power of the module can be adjusted to be a suitable value.
In the coupling lens of the fifth configuration, it is preferable that when the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction lens is xcex7, the following formula is satisfied:
30%xe2x89xa6xcex7xe2x89xa637%.
Further, it is more preferable that the following formula is satisfied:
30% less than xcex7 less than 37%.
In the coupling lens of the fifth configuration, it is preferable that a relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 25% and 50% of a relief period, and when the location of the apex is X and the depth of the relief function is W, W satisfies the following formula:
xe2x88x922.5X+1.66xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa6xe2x88x921.6X+1.8.
In the coupling lens of the fifth configuration, it is more preferable that a relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 25% and 50% of a relief period, and when the location of the apex is X and the depth of the relief function is W, W satisfies the following formula:
xe2x88x922.5X+1.75xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa6xe2x88x921.6X+1.71.
In the coupling lens of the fifth configuration, it is more preferable that a relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 25% and 50% of a relief period, and when the location of the apex is X and the depth of the relief function is W, W satisfies the following formula:
xe2x88x922.4X+1.67xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa6xe2x88x921.94X+1.86.
In the coupling lens of the fifth configuration, it is preferable that a relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 25% and 50% of a relief period, and when the location of the apex is X and the depth of the relief function is W, W satisfies the following formula:
xe2x88x922.4X+1.76xe2x89xa6W(nxe2x88x921)/xcexxe2x89xa6xe2x88x921.94X+1.77.
According to the above-mentioned respective preferable configurations, since the relief depth of the diffraction lens is adjusted suitably depending on the location of the apex, the light used for optical fiber coupling can be adjusted to have an intensity most suitable for a module.
In the coupling lens of the fifth configuration, it is preferable that a relief function of the diffraction lens has an approximately triangular shape with its apex at a location between 30% and 40% of a relief period.
In the coupling lens of the fifth configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
In the coupling lens of the fifth configuration, it is preferable that a material of the lens is resin and the diffraction lens is designed so that when a refractive index of the resin material and a wavelength of the semiconductor laser vary due to variation in temperature, the variation in focal length of the lens due to variation in the refractive index is corrected by the variation in focal length of the diffraction lens due to variation in the wavelength.
In the coupling lens of the fifth configuration, it is preferable that a material of the lens is resin, and when a focal length of the lens as a whole is f and a focal length of the diffraction lens is fd, the following formula is satisfied:
2 less than fd/f less than 5.
A semiconductor laser module according to a fourth configuration of the present invention includes, at least: a semiconductor laser; an optical fiber; a fixing member for fixing an incident end of the optical fiber; and a coupling lens for allowing a beam of light emitted from the semiconductor laser to form an image on the incident end of the optical fiber. The coupling lens is the coupling lens of the fifth configuration.
In the semiconductor laser module of the fourth configuration, the coupling lens of the present invention is used. Therefore, even when a module is constructed using a high output power laser source for high-speed communication, a suitable output power can be obtained, which does not exceed the upper limit output power prescribed in the ANSI standard and is at least the lowest output power required for optical fiber communication.
A coupling lens according to a sixth configuration of the present invention is used for coupling a beam of light emitted from a semiconductor laser to an optical fiber. The coupling lens is a pair of lenses of a refractive lens and a diffraction lens formed of concentric zones. The diffraction lens has a positive refracting power. When the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction lens is xcex7, the following formula is satisfied:
25%xe2x89xa6xcex7xe2x89xa640%.
Preferably, the following formula is satisfied:
25% less than xcex7 less than 40%.
Since the coupling lens of the sixth configuration is formed of the pair of lenses of the refractive lens and the diffraction lens, a lens formed by a photolithography technique can be used as the diffraction lens. Thus, the coupling lens can be formed at a low cost.
In the coupling lens of the sixth configuration, it is preferable that when the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction lens is xcex7, the following formula is satisfied:
30%xe2x89xa6xcex7xe2x89xa637%.
Further, it is more preferable that the following formula is satisfied:
30% less than xcex7 less than 37%.
In the coupling lens of the sixth configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
In the coupling lens of the sixth configuration, it is preferable that a material of the lens is resin and the diffraction lens is designed so that when a refractive index of the resin material and a wavelength of the semiconductor laser vary due to variation in temperature, the variation in focal length of the lens due to variation in the refractive index is corrected by the variation in focal length of the diffraction lens due to variation in the wavelength.
Since the coupling lens of such a preferable configuration is designed so that the variation in focal length due to variation in temperature of the refractive lens is corrected by the diffraction lens, an inexpensive resin material can be used as a material of the coupling lens, thus reducing the cost of the lens.
In the coupling lens of the sixth configuration, it is preferable that a material of the refractive lens is resin, and when a focal length of the lens as a whole is f and a focal length of the diffraction lens is fd, the following formula is satisfied:
2 less than fd/f less than 5.
In the coupling lens of the sixth configuration, it is preferable that a relief shape of the diffraction lens is formed of step-wise zones.
In the coupling lens of the sixth configuration, it is preferable that the diffraction lens is fixed with the centers of concentric zones of the diffraction lens being uncentered with respect to an optical axis of the refractive lens.
In the coupling lens of such a preferable configuration, since the diffraction lens is positioned so as to be uncentered with respect to the refractive lens, diffracted light can be separated in a direction of the optical axis and in a direction perpendicular to the optical axis. Therefore, when it is used in a semiconductor laser module, the processing accuracy of an aperture for intercepting unwanted light can be relaxed.
A semiconductor laser module according to a fifth configuration of the present invention includes, at least: a semiconductor laser; an optical fiber; a fixing member for fixing an incident end of the optical fiber; and a coupling lens for allowing a beam of light emitted from the semiconductor laser to form an image on the incident end of the optical fiber. The coupling lens is the coupling lens of the sixth configuration.
In the semiconductor laser module of the fifth configuration, a coupling lens formed of the above-mentioned pair of lenses is used and therefore a module can be constructed using an inexpensive lens.
In the semiconductor laser module of the fifth configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.
A semiconductor laser module according to a sixth configuration of the present invention includes, at least: a semiconductor laser, an optical fiber, a fixing member for fixing an incident end of the optical fiber, and a coupling lens for allowing a beam of light emitted from the semiconductor laser to form an image on the incident end of the optical fiber. A diffraction grating is positioned between the coupling lens and the optical fiber.
Since in the semiconductor laser module of the sixth configuration, a diffraction grating is positioned in an optical path, unwanted light can be separated on a surface of an aperture and therefore the processing accuracy of the aperture can be relaxed.
In the semiconductor laser module of the sixth configuration, it is preferable that when the diffraction efficiency of a diffracted light with an order used for optical fiber coupling out of diffracted lights from the diffraction grating is xcex7, the following formula is satisfied:
25%xe2x89xa6xcex7xe2x89xa640%.
Further, it is more preferable that the following formula is satisfied:
25% less than xcex7 less than 40%.
In the semiconductor laser module of the sixth configuration, it is preferable that a wavelength xcex of the semiconductor laser satisfies the following formula:
700 nm less than xcex less than 1400 nm.